LEDs are current-controlled devices in the sense that the intensity of the light emitted from an LED is related to the amount of current driven through the LED. FIG. 1 shows a typical relationship of relative luminosity to forward current in an LED. The longevity or useful life of LEDs is specified in terms of acceptable long-term light output degradation. Light output degradation of LEDs is primarily a function of current density over the elapsed on-time period. LEDs driven at higher levels of forward current will degrade faster, and therefore have a shorter useful life, than the same LEDs driven at lower levels of forward current. It therefore is advantageous in LED lighting systems to carefully and reliably control the amount of current through the LEDs in order to achieve the desired illumination intensity while also maximizing the life of the LEDs.
LED illumination products have been developed which provide the ability to vary the forward current through the LEDs over an acceptable range in order to provide dimming capability. LED lighting systems have also been devised which, through the use of multiple colors of LEDs and individual intensity control of each color, can produce a variety of color hues. Systems incorporating Red, Green, and Blue LEDs can achieve near infinite color variations by varying the intensity of the Red, Green, and Blue color banks.
As LED Lighting Systems have become more prevalent, various methods have been devised to control the current driven through the LEDs to achieve dimming and color mixing. One common method is a Pulse Width Modulation (PWM) scheme such as that set forth in U.S. Pat. Nos. 6,618,031, 6,510,995, 6,150,774, 6,016,038, 5,008,595, and 4,870,325, all of which are incorporated herein by reference as if set forth in full. PWM schemes pulse the LEDs alternately to a full current “ON” state followed by a zero current “OFF” state. The ratio of the ON time to total cycle time, defined as the Duty Cycle, in a fixed cycle frequency determines the time-average luminous intensity. Varying the Duty Cycle from 0% to 100% correspondingly varies the intensity of the LED as perceived by the human eye from 0% to 100% as the human eye integrates the ON/OFF pulses into a time-average luminous intensity.
Although PWM schemes are common, there are several disadvantages to this method of LED intensity control. The fixed frequency nature of PWM means that all LEDs switch on (to maximum power draw) and off (zero power draw) at the same time. Large illumination systems can easily require several amperes of current to be instantaneously switched on and off. This can create two problems. First, the rapid on and off switching of the system can create asymmetric power supply loading. Second, the pulsing of the current through electrical leads can create difficult to manage electromagnetic interference (EMI) problems because such leads may act as transmitters of radiofrequency energy that may interfere with other devices operating at similar frequencies.
In order to address these problems with PWM, an alternate method of LED intensity control, called Frequency Modulation (FM) has been developed and implemented by Artistic Licence Ltd. and described at their website, particularly in Application Note 008, located at http://www.artisticlicence.com/ (last visited Jun. 17, 2004).
The FM method of LED intensity control is similar to the PWM method in that the LEDs are switched alternately from a maximum current state to a zero current state at a rate fast enough for the human eye to see one integrated time-average intensity. The two methods differ in that PWM uses a fixed frequency and a variable pulse width (duty cycle), whereas FM delivers a fixed width pulse over a variable frequency. Both of these methods achieve a dimming effect through the varying ratio of LED ON time to OFF time. Where the FM method improves upon the PWM method, is in the fact that a varying frequency creates fewer EMI problems, and reduces the asymmetric power supply loading effect.
The FM method, however, suffers from the same drawbacks of the PWM method when the dimming level is held constant, or is changing at a relatively slow rate. In fact, at a constant level of dimming, it can be seen that the EMI and asymmetric power supply loading effects of PWM and FM are identical. As the size of the lighting system (total number of LEDs) controlled by a central control and power supply gets large, these negative effects can get correspondingly large and difficult to overcome.
There is a third prior art method of LED intensity control that eliminates the drawbacks of the PWM and FM techniques, called Analog Control. Analog Control is a method of varying the current being driven through the LEDs through a continuous analog range from zero through the maximum desired level. Since the LEDs are not constantly pulsed between two states of zero and maximum current, EMI problems are minimized, as are power supply loading problems associated with large instantaneous changes in power draw.
The Analog Control method, although solving the problems associated with PWM and FM techniques for LED driving, nevertheless has other drawbacks. Due to process variations and tolerances of analog components, including the LEDs themselves, variations in luminous intensity from the desired intensity, i.e., brightness control inaccuracies, can show up at lower levels of current where component tolerances make up a larger percentage of the total effect. In addition, wavelength shifts can occur especially at lower current levels, which can lead to undesired color shifts in the light output by the LEDs. As lighting designers seek to employ very low levels of output illumination, a higher degree of control in this range becomes more and more desirable.
It is desirable then, to devise a circuit for variably controlling the current through LEDs without the drawbacks inherent in PWM and FM schemes, and that overcomes the problems with the Analog Control circuit associated with low current levels that are described above. The invention described herein solves these problems effectively while remaining simple and inexpensive to implement.